The Tennessee Stormwater Runoff Assessment Tool (RRAT)

July 13, 2015

By Lee Gentry, P.E.

On June 18, 2015, the University of Tennessee SMART Center and TDEC conducted the inaugural design training course. The course was for the new Stormwater Runoff Reduction Assessment Tool (RRAT). I was excited to participate in the course held in Knoxville, Tennessee. The class was coordinated by Mr. Tim Gangaware, and was taught by Dr. Andrea Ludwig and Dr. John Buchanan.

In general, the model is a tool for assessing the effects of various green infrastructure and other site features on Stormwater runoff and infiltration. The model was developed primarily by Dr. Daniel Yoder. The user interface is intuitive and easy to follow, while the analytics are robust and very flexible. It’s hard to imagine a situation that can’t be realistically represented in the model. In addition, the model has been tailored to test the results against the State of Tennessee’s Phase II Stormwater Discharge NPDES General Permit requirements for infiltration and treatment. It can also test the results for situations associated with karst, hotspots, existing infiltration limits, and other issues.

RRAT blog

Typical Bio-Retention cell for a large parking lot. The cell infiltrates much of the runoff from the paved areas, resulting in less runoff from the site.

As Cities and Counties in the MS4 program progress in requiring runoff reduction in new developments, the RRAT is a practical tool when designing a site to meet those requirements. LDA Engineering has been involved in workshops and other testing of the model since its early stages. From this, we are well versed in its use and application.

We are also skilled in using several other methods that are currently being used or proposed. LDA is also developing an approach, partially based on the research behind RRAT, that has the advantage of being able to prepare a runoff reduction analysis and site drainage design all in one model. We expect to roll that out for consideration in late 2015 or early 2016.

In the meantime, we like to talk about this stuff. Give us a call if you have some thoughts on the issue or need some help.